Title:
Displacing entanglement back and forth between the micro and macro domains

Abstract: Quantum theory is often presented as the theory describing the microscopic
world, and admittedly, it has done this extremely well for decades.
Nonetheless, the question of whether it applies at all scales and in particular
at human scales remains open, despite considerable experimental effort. Here,
we report on the displacement of quantum entanglement into the domain where it
involves two macroscopically distinct states, i.e. two states characterised by
a large enough number of photons to be seen, at least in principle, with our
eyes and that could be distinguished using mere linear - coarse-grained -
detectors with a high probability. Specifically, we start by the generation of
entanglement between two spatially separated optical modes at the single photon
level and subsequently displace one of these modes up to almost a thousand
photons. To reliably check whether entanglement is preserved, the mode is
re-displaced back to the single photon level and a well established
entanglement measure, based on single photon detection, is performed. The
ability to displace an entangled state from the micro to the macro domain and
back again provides a fascinating tool to probe fundamental questions about
quantum theory and holds potential for more applied problems such as quantum
sensing.

Comments:

7 pages, 4 figures. Slightly different from the published version. See arXiv:1212.3710 for the initial theoretical proposal. See also arXiv:1306.0843 for the corresponding measure of macroscopicity